It is old, as disclosed in U.S. Pat. No. 3,045,760, to provide a wireline retrievable differential pressure operated safety valve adjusted to close at a predetermined rate of flow. The predetermined rate of flow may be adjusted by changing the coil spring, adjusting the number of spring spacer rings used, or changing the size of the choke.
Generally, well production changes with time and it is desirable to change or adjust the valve to close at a different rate of flow as the well production changes. This can be accomplished easily with the wireline retrievable safety valve of the prior art which is releasably locked in the well tubing and can be removed and adjusted.
However, it would be desirable to utilize a tubing retrievable safety valve, that is, one which is threaded into and forms part of the well tubing because the tubing retrievable type valve provides a larger through bore. However, the closing force of a tubing retrievable safety valve can not easily be changed except by pulling the well tubing which is undesirable and is not commercially feasible.
The present invention is directed to an improved tubing differential pressure actuated tubing retrievable well safety valve in which the closing forces can be adjusted from the well surface as the need arises.